The present invention relates to methods and apparatuses for fabricating semiconductor devices and more particularly relates to a method for forming a very thin gate insulating film having a high dielectric constant.
Recently, with a great progress in a high degree of integration of semiconductor integrated circuits, attempts have been made at reducing the size of transistor devices and increasing the performance thereof in MOS semiconductor devices. Specifically, as the size of devices such as transistors has been reduced, achievement of highly reliable MOS devices is required. In order to increase the reliability of MOS devices, each member composing a MOS device should be highly reliable. Specifically, as for gate insulating films used in MOS devices, the thickness thereof has been rapidly reduced and it is expected that very thin gate insulating films having a thickness of 2 nm or less will be commonly used during the 21st century. As the thickness of a gate insulating film becomes thinner, a higher degree of uniformity in properties of the gate insulating film is required. Therefore, the achievement of gate insulating films having excellent properties has been regarded as important so that properties of a gate insulating film are said to determine characteristics of a MOS transistor and even electric properties of a semiconductor integrated circuit.
Currently, SiO2 films are used as gate insulating films in most cases. When an SiO2 film is used as a gate insulating film, there have been big problems, in very thin regions having a thickness of 2 nm or less, of reduced reliability of the insulating film and increased gate leakage current which is directly caused by a tunnel phenomenon current. In fact, the gate leakage current has to be suppressed to achieve an LSI having low power consumption. However, such gate leakage current due to the tunnel phenomenon is a phenomenon that is governed by physical rules and is strongly influenced by the physical characteristics of SiO2 films. Therefore, it is no longer possible to achieve the LSIs having low power consumption by the very thin SiO2 films.
Then, there have been increased efforts to adopt a film having a higher dielectric constant than the SiO2 film, thereby ensuring as great a capacitance as that of the very thin SiO2 film while increasing the thickness of the insulating film to suppress leakage current, for example, as described in a reference material (Lee et al. IEEE/International Electron Device Meeting 99, p. 133). For example, by using an HfO2 film (a high dielectric film) as a gate insulating film, it is possible to achieve a transistor which has a capacitance corresponding to a thickness of 2 nm or less in terms of the SiO2 film and which occurs three orders of smaller leakage current than the SiO2 film. For example, if the gate insulating film is composed of HfO2, a method for depositing films using reactive sputtering is generally adopted.
FIGS. 18(a) through 18(c) are cross-sectional views illustrating process steps of forming a gate insulating film composed of an HfO2 film by a plasma CVD method.
A p-type Si substrate 501 shown in FIG. 18(a) is first prepared, and then in the process step shown in FIG. 18(b), an HfO2 target is bombarded with Ar ions to sputter the Hf atoms, thereby injecting the Hf atoms into an Ar plasma 504. An HfO2 activated species 503 is generated in the Ar plasma 504, and the HfO2 activated species 503 is deposited on the Si substrate 501 to form an HfO2 film 502. Then, in the process step shown in FIG. 18(c), a polysilicon film 505 is deposited on the HfO2 film 502. Thereafter, the polysilicon film 505 and the HfO2 film are patterned, and thereby a gate insulating film and a gate electrode can be formed, though they are not shown.
As for system LSIs, LSIs in which multiple types of MIS devices including gate insulating films with different thicknesses are mounted are utilized. Conventionally, an LSI in which two types of devices, i.e., an MIS device of a 3.3 V system including a thick gate insulating film and provided in an I/O section and an MIS device including a thin gate insulating film provided in a logic core section, are mounted has been well known. Nowadays, however, the thin gate oxide film being provided in the logic core section is subdivided into more types, and in order to reduce the gate leakage current in a stand-by state, an MIS device in which the thickness of an gate insulating film has been physically increased is provided only in a specific part of an LSI. In this case, a film forming method in which ions, such as F ions, have been implanted in advance into a substrate region on which a thick gate insulating film is to be formed, and gate insulating films having different thicknesses are simultaneously formed on the same substrate by using the difference in oxidation speed between the ion-implanted region and the remaining region, or like methods are adopted.
Problems to be Solved
It has been pointed that when the HfO2 film or a ZrO2 film which has a high dielectric constant is used as the gate insulating film of a MOS transistor, the interface between the gate insulating film and an Si substrate has poor properties relative to the case of using an SiO2 film and therefore that characteristics of the MOS transistor can not be expectedly improved. In the case of a sputtering method concerns arise about damages to the Si substrate 501 due to ion attacks. Thus, there have been more cases in which not the HfO2 film or the ZrO2 film but so-called silicate materials such as HfSiO-based materials or ZrSiO-based materials are introduced. However, the silicate materials have problems, such as difficulties in controlling the composition of a film containing a silicate material or great difficulties in controlling the thickness of an SiO2 layer around the surface of the Si substrate in forming a silicate layer. Accordingly, when a silicate material is used as the gate insulating film, the level of the gate leakage current varies widely, and therefore the silicate material is not suitable for mass production.
Furthermore, when multiple types of gate insulating films composed of an oxide film (or an oxynitride film) and having different thicknesses are provided on a substrate in accordance with desired characteristics of the transistor, it is difficult to control the thicknesses of the insulating films with high accuracy in the known method in which the oxidation speed is varied by implanting F ions, and therefore the capacitance between the gate and the substrate unwantedly varies widely. On the other hand, when a step of partially removing the oxide film is repeatedly performed to form three oxide films or oxynitride films having different thicknesses, problems caused by etching a silicon substrate surface for many times might become noticeable.
An object of the present invention is to provide a method for easily forming a gate insulating film in which good properties of an SiO2 film on an Si substrate is maintained and which has a high dielectric constant, and a method for fabricating a semiconductor device including the gate insulating film so as to accommodate size reduction of transistors.
A first method for forming an insulating film according to the present invention includes the steps of: a) introducing at least oxygen into a surface region of a semiconductor substrate, thereby forming an initial insulating film; b) introducing at least one type of metal into at least part of the initial insulating film; and c) performing heat treatment to diffuse the metal in the initial insulating film, thereby forming at least one high dielectric film having a higher dielectric constant than the initial insulating film on at least part of the semiconductor substrate.
According to the method, a metal that has been introduced into at least part of an initial insulating film is diffused, thereby forming from the initial insulating film a high dielectric film having a high dielectric constant. Thus, for example, it is possible to increase the thickness of the initial insulating film without changing the capacitance of the entire initial insulating film and in such a case the leakage current can be reduced. Moreover, in the case where the capacitance of the high dielectric film is increased to become larger than that of the entire initial insulating film, a good state at the interface between the initial insulating film formed by introducing oxygen into a semiconductor substrate and the semiconductor substrate can be maintained. Accordingly, an insulating film which has a good interface state between the insulating film and the semiconductor substrate and excellent leakage properties can be achieved by relatively simple process steps.
In the step a), a silicon substrate can be used as the semiconductor substrate, and a silicon oxide film or a silicon oxynitride film can be formed as the initial insulating film.
In the step b), the metal injected is at least one metal selected from the group consisting of Hf, Zr and Al. Thereby, an insulating film having a particularly high dielectric constant can be achieved.
In the step c), an SiO2 film may be left in a region around the interface between the initial insulating film and the semiconductor substrate.
In the step b), metal ions in a plasma may be introduced into the initial insulating film. Thereby, metal is effectively introduced into the initial insulating film.
In the step b), the metal ions may be introduced into the initial insulating film by an ion implantation method.
In the step b), the metal may be introduced into only a first transistor formation region in the initial insulating film and in the step c), the high dielectric film may be formed in only the first transistor formation region in the initial insulating film, whereby the high dielectric film is used as a gate insulating film of a first transistor and the initial insulating film is used as a gate insulating film of a second transistor which operates at a slower speed than the first transistor. For example, in a semiconductor integrated circuit, the first transistor may be used as a transistor which is required to operate at a high speed and the second transistor may be used as a transistor which is required to have high reliability.
In the step b), different types of multiple metals, as said at least one type of metal, may be introduced into multiple regions in the initial insulating film, and in the step c), the metals may be diffused in the initial insulating film to form as said at least one high dielectric film multiple high dielectric films having a higher dielectric constant than the initial insulating film, and the high dielectric films may be used as gate insulating films of multiple transistors having different operation characteristics. In this manner, multiple types of transistors having different properties (e.g., threshold voltage) can be provided on the semiconductor substrate without changing the thicknesses of the gate insulating films unlike the known method. An example of such cases is the case where a transistor in an I/O section, a transistor which is required to operate at a high speed and a transistor which can operate at a relatively slow speed and has a low threshold voltage are mounted.
In the step b), one type of metal may be introduced into multiple regions in the initial insulating film at different concentrations and in the step c), the metal may be diffused in the initial insulating film to form as said at least one high dielectric film multiple high dielectric films having a higher dielectric constant than the initial insulating film. In this manner, also, multiple types of transistors having different properties (e.g., threshold voltage) can be provided on the semiconductor substrate.
A second method for forming an insulating film according to the present invention includes the steps of a) forming an initial insulating film including at least a metal oxide film on a semiconductor substrate; b) injecting atoms of the semiconductor forming the semiconductor substrate into at least part of the initial insulating film; and c) performing heat treatment to diffuse the atoms of the semiconductor in the initial insulating film, thereby forming at least one dielectric film having a different dielectric constant from the initial insulating film in at least part of the initial insulating film.
According to the method, a dielectric film is formed from a metal oxide film having a high dielectric constant so that a dielectric film having a higher dielectric constant than a known thermal oxide film is obtained. Therefore, a dielectric film having excellent leakage properties can be obtained in a relatively simple manner.
In the step c), the heat treatment may be performed in an oxidizing atmosphere. In this manner, the surface portion of the semiconductor substrate is thermally oxidized so that the rate of contained semiconductor atoms, i.e., O atoms increases in part of the dielectric film which is closer to the semiconductor substrate, resulting in a good interface state between the dielectric film and the semiconductor substrate.
It is preferable that in the step a), a silicon substrate is used as the semiconductor substrate and that a layered film selected from the group consisting of layered films of an SiO2 film and a ZrO2 film, an SiO2 film and an HfO2 film, and a ZrO2 film and an HfO2 film is formed as the initial insulating film.
In the step b), the atoms of the semiconductor may be introduced into multiple regions in the initial insulating film at different concentrations and in the step c), multiple dielectric films having different dielectric constants may be formed as said at least one dielectric film, wherein the dielectric films may be used as gate insulating films of multiple transistors having different operation characteristics. In this manner, multiple types of transistors having different properties (e.g., threshold voltage) can be provided on the semiconductor substrate without changing the thicknesses of the gate insulating films unlike the known method. An example of such cases is the case where a transistor in an I/O section, a transistor which is required to operate at a high speed and a transistor which can operate at a relatively slow speed and has a low threshold voltage are mounted.
A third method for forming an insulating film according to the present invention includes the steps of: a) introducing at least oxygen into a surface region of a semiconductor substrate, thereby forming an initial insulating film; b) introducing at least one type of metal into at least part of the initial insulating film; c) forming a conductor film on the initial insulating film; d) patterning the conducting film, thereby forming a gate electrode of at least one transistor; and e) introducing an impurity into regions of the semiconductor substrate located on both sides of the gate electrode, thereby forming source/drain regions of at least one transistor; and f) performing heat treatment to activate the impurity which has been introduced into the source/drain regions and to diffuse the metal in the initial insulating film, thereby forming in at least part of the initial insulating film a high dielectric film which has a higher dielectric constant than the initial insulating film and which serves as a gate insulating film of at least one transistor.
According to the method, a metal that has been introduced into at least part of an initial insulating film is diffused, thereby forming a high dielectric film having a high dielectric constant from the initial insulating film, in the same manner as according to the method for forming an initial insulating film. Thus, for example, it is possible to increase the thickness of the initial insulating film without changing the capacitance of the entire initial insulating film, resulting in the reduction of leakage current. Moreover, in the case where the capacitance of the high dielectric film is increased to become larger than that of the entire initial insulating film, a good state at the interface between the initial insulating film formed by introducing oxygen into a semiconductor substrate and the semiconductor substrate can be maintained. Accordingly, an insulating film which has a good interface state between the insulating film and the semiconductor substrate and excellent leakage properties can be achieved by a relatively simple process steps.
In the step a), a silicon substrate may be used as the semiconductor substrate and a silicon oxide film or a silicon oxynitride film may be formed as the initial insulating film.
In the step b), different types of multiple metal films may be deposited on multiple regions in the initial insulating films and in the step c), multiple high dielectric films having different dielectric constants may be formed as said at least one high dielectric film, wherein the high dielectric films may be used as gate insulating films of multiple transistors having different operation characteristics. In this manner, multiple types of transistors having different properties (e.g., threshold voltage) can be provided on the semiconductor substrate without changing the thicknesses of the gate insulating films unlike the known method. An example of such cases is the case where a transistor in an I/O section, a transistor which is required to operate at a high speed and a transistor which can operate at a relatively slow speed and has a low threshold voltage are mounted.
A first method for fabricating a semiconductor device according to the present invention includes the steps of: a) introducing at least oxygen into a surface region of a semiconductor substrate, thereby forming an initial insulating film; b) introducing at least one type of metal into at least part of the initial insulating film; c) forming a conductor film on the initial insulating film; d) patterning the conductor film, thereby forming a gate electrode of at least one transistor; and e) introducing an impurity into regions of the semiconductor substrate located on both sides of the gate electrode, thereby forming source/drain regions of at least one transistor; and f) performing heat treatment to activate the impurity which has been introduced into the source/drain regions, and to diffuse the metal in the initial insulating film, thereby forming in at least part of the initial insulating film a high dielectric film which has a higher dielectric constant than the initial insulating film and which serves as a gate insulating film of at least one transistor.
According to the method, a semiconductor device including an insulating film, which is formed according to the initial insulating film formation method and has a good interface state between the insulating film and the semiconductor substrate and excellent leakage properties, can be achieved.
In the step b), the metal may be introduced into only a first transistor formation region in the initial insulating film and in the step f), the high dielectric film may be formed in only the first transistor formation region in the initial insulating film, wherein, as said at least one transistor, a first transistor in which the high dielectric film serves as a gate insulating film, and a second transistor in which the initial insulating film serves as a gate insulating film and which operates at a slower speed than the first transistor may be formed. Thereby, for example, in a semiconductor integrated circuit, the first transistor can be used as a transistor which is particularly required to operate at a high speed and the second transistor can be used as a transistor which is required to have high reliability.
In the step b), different types of multiple metals, as said at least one type of metal, may be introduced into multiple regions in the initial insulating film, and in the step f), the metals may be diffused in the initial insulating film to form as said at least one high dielectric film multiple high dielectric films having a higher dielectric constant than the initial insulating film, wherein multiple transistors including the high dielectric films which serve as gate insulating films and having different operation characteristics may be formed.
A second method for fabricating a semiconductor device according to the present invention includes: a) forming an initial insulating film including at least a metal oxide film on a semiconductor substrate; b) injecting atoms of the semiconductor forming the semiconductor substrate into at least part of the initial insulating film; c) forming a conductor film on the initial insulating film; d) patterning the conductor film, thereby forming a gate electrode of at least one transistor; e) introducing an impurity into regions of the semiconductor substrate located on both sides of the gate electrode, thereby forming source/drain regions of at least one transistor; and f) performing heat treatment to activate the impurity which has been introduced into the source/drain regions, and to diffuse the atoms of the semiconductor in the initial insulating film, thereby forming in at least part of the initial insulating film a dielectric film which has a different dielectric constant from the initial insulating film and which serves as a gate insulating film of at least one transistor.
According to the method, a semiconductor device including an insulating film, which has a good interface state between the insulating film and the semiconductor substrate and excellent leakage properties, can be achieved.
In the step b), the atoms of the semiconductor may be introduced into multiple regions in the initial insulating film at different concentrations, and in the step c), multiple dielectric films having different dielectric constants may be formed as said at least one dielectric film, wherein multiple transistors including the high dielectric films which serve as gate insulating films and having different operation characteristics may be formed as said at least one transistor.
A third method for fabricating a semiconductor device according to the present invention includes the steps of: a) forming an initial insulating film on a semiconductor substrate; b) depositing a metal film on at least part of the initial insulating film; c) forming a conductor film on the metal film; d) patterning the conductor film, thereby forming a gate electrode; e) introducing an impurity into regions of the semiconductor substrate located on both sides of the gate electrode, thereby forming source/drain regions; and f) performing heat treatment to activate the impurity which has been introduced into the source/drain regions, and to diffuse metal atoms of the metal film in the initial insulating film, thereby forming in at least part of the initial insulating film a high dielectric film which has a higher dielectric constant than the initial insulating film and which serves as a gate insulating film of at least one transistor.
According to the method, also, a semiconductor device including an insulating film, which has a good interface state between the insulating film and the semiconductor substrate and excellent leakage properties, can be achieved.
In the step b), different types of multiple metal films may be deposited on multiple regions in the initial insulating films, and in the step f), multiple dielectric films having different dielectric constants may be formed as said at least one dielectric film, wherein multiple transistors including the high dielectric films as gate insulating films and having different operation characteristics may be formed.